Censorship and hentai/lolikon

Until recently, japanese law required that all erotic material shown publicly be censored.

There were different attempts to bypass this old law:

Changing the color of the genitalia, argumenting that the item shown is not genitalia

Only showing the genitals in very close detail, so that it's not definitely recognizable

Substituting the genitalia with non-sexual objects that look similar, eg. a penis with a banana

Making the male subjects non-human, thus argumenting that their anatomy is different (See tentacles)

With time, the prosecutions became more rare, and the artists made the censure more and more revealing. Doujinshi and manga artists often included only the the minimal censorship necessary to satisfy the laws, such as a white or black bar over the opening of the vagina. The results were often laughable, and many questioned the necessity of the censorship. Though the laws are no longer in place, most artists still censor their work out of tradition or fear of another law change.

Censorship of pornography in japan led to the creation of, among other genres, tentacle porn and bukkake, in attempts to make pornography more extreme.